Table 2 The independent association between serum cotinine levels, BMI groups and asthma, and interaction between serum cotinine levels and BMI on asthma

Serum cotinine¹

 Low-level

Ref

Ref

Ref

 High-level

1.298 (1.104–1.527)

0.002

1.330 (1.129–1.565)

0.001

1.190 (1.004–1.410)

0.045

BMI groups

 Croup A

Ref

Ref

Ref

 Croup B

1.305 (1.110–1.535)

0.002

1.274 (1.081–1.503)

0.004

1.275 (1.079–1.506)

0.005

 Croup C

1.731 (1.436–2.087)

 < 0.001

1.663 (1.378–2.006)

 < 0.0001

1.636 (1.354–1.977)

 < 0.001

Interaction

    Low-level serum cotinine and Group A

Ref

Ref

Ref

 Low-level serum cotinine and Group B

1.360 (1.131–1.635)

0.001

1.330 (1.105–1.602)

0.003

1.329 (1.102–1.602)

0.003

 Low-level serum cotinine and Group C

1.758 (1.440–2.145)

 < 0.001

1.693 (1.389–2.063)

 < 0.0001

1.670 (1.367–2.041)

 < 0.001

 High-level serum cotinine and Group A

1.332 (1.054–1.684)

0.017

1.371 (1.083–1.738)

0.009

1.199 (0.936–1.537)

0.150

 High-level serum cotinine and Group B

1.549 (1.175–2.043)

0.002

1.539 (1.162–2.038)

0.003

1.359 (1.009–1.829)

0.043

 High -level serum cotinine and Group C

2.125 (1.574–2.867)

 < 0.001

2.065 (1.528–2.789)

 < 0.0001

1.822 (1.357–2.448)

 < 0.001

Interaction

 Attributable proportion of interaction (95%CI)

0.258 (0.083–0.433)

0.338 (0.234–0.443)

0.206 (0.075–0.337)

 Synergy index (95%CI)

1.934 (1.344–2.783)

2.225 (1.813–2.732)

1.617 (1.126–2.098)